His gf has asked him to not live the lifestyle he has grown to love. I fear this will cause resentment and could cause failure. Am I wrong in worrying about this?

It sounds like she may be what is sometimes referred to as a "cowgirl" - a person who gets involved with someone in a poly or open relationship with the intentions of getting them to give up their other partner(s) and be monogamous with the cowgirl/boy.